This invention relates to a photoresist for an optical disc particularly capable of being preferably utilized for manufacturing an optical disc having high recording density and also relates to a method of preparing an optical disc by utilizing the photoresist.
In a conventional art, an optical disc is generally manufactured in accordance with the following steps.
First, a positive-type photoresist is coated with uniform thickness on a glass substrate which was ground finely to thereby form a photoresist layer. In the next step, a laser beam is irradiated to a portion of the photoresist layer at which a pit is formed. A portion of the photoresist layer irradiated to the laser beam is easily dissolved in an alkali liquid developer in comparison with another portion not irradiated. Accordingly, when the development is performed by coating the alkali liquid developer on the photoresist layer, a recess corresponding to a pit is formed to the portion irradiated to the laser beam. Thereafter, a metallic film is formed on the photoresist layer to which an electro conductivity is imparted, and in this step, for example, silver (Ag) or nickel (Ni) may be utilized as a metal for forming the metallic film. In the next step, an electrocasting treatment is performed to form an electrocast product, Ni electrocast, for example, on the metallic film. The nickel electrocast is then peeled off from the glass substrate and the photoresist layer together with the metallic film, the thus peeled off nickel electrocast and metallic film being called a metallic original, i.e. stamper.
In such optical disc original manufacturing steps, a photoresist for manufacturing a semiconductor is utilized as it is in the conventional art.
However, in recent studies and experiments of the inventors of the subject application for achieving high density of an optical disc, it was found that the conventional photoresist for manufacturing a semiconductor is not suitable for the manufacturing of the optical disc having high recording density.
Namely, in a case where an optical disc original having high density with small pit size is manufactured by utilizing a conventional photoresist for manufacturing a semiconductor and the optical disc is formed from this optical disc original, a noise level increases at a reproduction time in accordance with the reduction of the pit size and, hence, a sufficient CN ratio (carrier-wave/noise ratio), this providing a significant problem.